1. Field of the Invention
The present invention relates to a phosphor, a radiation image recording and reproducing method utilizing the same, and a radiation image storage panel employing the same. More particularly, the invention relates to a divalent europium activated alkaline earth metal halide stimulable phosphor, a radiation image recording and reproducing method utilizing the same, and a radiation image storage panel employing the same.
2. Description of the Prior Art
There is well known a divalent europium activated alkaline earth metal fluorohalide phosphor (M.sup.II FX:Eu.sup.2+, in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; and X is a halogen other than fluorine), as a divalent europium activated alkaline earth metal halide phosphor. The phosphor gives emission (spontaneous emission) in the near ultraviolet region when exposed to a radiation such as X-rays. The phosphor also gives emission (stimulated emission) in the near ultraviolet region when excited with an electromagnetic wave such as visible light or infrared rays after exposure to a radiation such as X-rays. Namely, the phoshor is a stimulable phosphor.
A radiation image recording and reproducing method utilizing a sitmulable phosphor can be employed in place of the conventional radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and an intensifying screen as described, for instance, in U.S. Pat. No. 4,239,968. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value, especially when the method is used for medical diagnosis.
As for a stimulable phosphor employable in the radiation image recording and reproducing method, almost no stimulable phosphor other than the above-mentioned divalent europium activated alkaline earth metal fluorohalide phosphor has been known.
The present inventors discovered a novel divalent europium activated alkaline earth metal halide phosphor having the following formula: EQU M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+
in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I, and X.noteq.X'; and a and x are numbers satisfying the conditions of 0.1.ltoreq.a .ltoreq.10.0 and 0&lt; x.ltoreq.0.2, respectively, and applied for a patent with respect to said phosphor, a radiation image recording and reproducing method utilizing said phosphor and a radiation image storage panel employing said phosphor (U.S. patent application Ser. No. 660,987 and European patent application No. 84112417.5).
The novel divalent europium activated alkaline earth metal halide phosphor has been confirmed to have a crystal structure different from that of the aforementioned M.sup.II FX:Eu.sup.2+ phosphor on the basis of the X-ray diffraction patterns as described in the above application. This phosphor gives spontaneous emission (peak wavelength: approx. 405 nm) in the near ultraviolet to blue region when exposed to a radiation such as X-rays, ultraviolet rays and cathode rays, and also gives stimulated emission in the near ultraviolet to blue region when excited with an electromagnetic wave having a wavelength within the region of 450-1000 nm after exposure to a radiation such as X-rays, ultraviolet rays and cathode rays. Accordingly, the phosphor is very useful as a phosphor for a radiation image storage panel employed in the radiation image recording and reproducing method utilizing a stimulable phosphor.
The sensitivity of the radiation image recording and reproducing method employing the radiation image storage panel to a radiation generally increases as the luminance of stimulated emission of the phosphor employed therefor increases. Accordingly, the stimulable phosphor employed for the panel is desired to show the stimulated emission of luminance as high as possible.